HELGOL,~NDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 49, 519-528 (1995) Macroalgal mass development in the Wadden Sea: iirst experiences with a monitoring system K. Kolbe l, E. Kaminski 2, H. Michaelis 1, B. Obert 1 & J. Rahmel 1 llVieders~chsisches Landesamt fDr Okologie, Forschungsstelle KDste; An der Mfihle 5, D-26548 Norderney, Germany 2[nstitut fDr Mfeereskunde (Abtlg. 1Vleeresbotanik), Universit~t Kiel; DDsternbrooker Weg 20, D-24105 Kiel, Germany ABSTRACT: The distribution and cover density of macroalgae (Chlorophyta, Ulvaceae) were estimated by means of aerial surveys in 1990-1992 in the Wadden Sea of Niedersachsen, an intertidal area of some 1200 km 2 situated at the German North Sea coast. Each year, up to a maximum of 15 % of the total area was covered by algae. The spatial distribution was hetero- geneous. In some subregions the macroalgal carpets covered from 30 % up to 60 % of the tidal flats. The cover density was at its peak in 1990. Additionally, tentative ground truth investigations were carried out on species composition. Reviewing other reports of macroalgal mass development at various sites in Europe, it is assumed that in the German Wadden Sea the recent macroalgal blooms have to be regarded as a response to eutrophication, and will presumably remain a chronic problem for many years to come. INTRODUCTION In the summer of 1989 attention was drawn to a sudden outburst of benthic green algae (Ulvaceae) occurring simultaneously in the entire German Wadden Sea. Large parts of the tidal flats were covered by macroalgal vegetation, a hitherto unfamiliar occurrence, which varied from a scattered patchwork, to dense carpets of up to 25 cm thickness. Air-surveillance was commenced by both Schleswig-Holstein (Reise, pers. commun.) and Niedersachsen. In the following summers (up to 1992) the green algae have repeated their excessive growth. Accordingly, the aerial surveys have been main- tained and have meanwhile resulted in a regular monitoring programme. The results for 1990-1992 from the Niedersachsen coast, i.e. the area between the Eros and the Elbe estuaries, are given in the present report. The current situation is compared with early descriptions of the Wadden Sea vegetation (Nienburg, 1927; Linke, 1939; Gessner, 1957) and some recent studies on macroalgal mass development. METHODS Aerial surveys Aerial surveys of algal cover along the coast of Niedersachsen (Fig. 1) were carried out 3 to 5 times during the summers of 1990-1992. They were performed with a single- ' Biotogische Anstalt Helgoland, Hamburg 520 K. Kolbe, E. Kaminski, H. Michaelis, B. Obert & J. Rahmel N "'- .~| NORTH SEA i. =SI. e 5 6 ".j Bremerhavefl Wilhelmshaven ~0 ~,~=...~ Emden NL ~ FRG ~" ~ ~ 1'o iokm Fig. 1. The survey area (shaded), divided into 16 subregions from west to east for data interpretation purposes engined aeroplane from a constant height of 200 m. The mappings were done at about low tide when the intertidal flats were completely exposed. Each time, two flights were necessary to cover the whole Niedersachsen coast. Topographic maps (1:50 000) were used for the plots. As standard maps do not represent the dynamic morphology of the tidal flats and the channel system properly, they were corrected in advance and com- pleted by information from the most recent aerial photographs available, Mapping was done independently by two trained persons, observing the area through the windows of the aeroplane. The localized intertidal algal stands were plotted manually onto the maps. Different cover densities ( 1-5 %, 6-20 %, 21-50 %, 51-80 %, > 80 %), estimated from visual impression of colour and texture, were indicated by different hachures. Varying thicknesses of the algal mats could not be distinguished from the air. The final result was gained by comparison, combination, and interpolation of the two parallel plots. The areas of each cover scale were measured from the resulting maps using a digitizer. The share of the tidal flats covered by algae was calculated for each subregion and for the entire area (1200 km2). The validity of the cover density assessments was examined in July 1991 by local ground truth observations in 7 localities near the islands Borkum, Spiekeroog, and Mellum (subregions 1, 8 and 12). Again the cover density was estimated visually. In five cases the estimations were well in accordance with the assessments from the aerial survey which was performed 7 to 10 days in advance. In one case the aerial estimation was higher, in one case lower than the ratings on the spot. Because of these differences, only two categories, i.e. 1-50 % and >50 %, were used in the final evaluation (Fig. 2). Macroalgae in the Wadden Sea 521 ~ ~r.~ ~ cover category 60 >50% .1111 40 i/.,// ..... 20 (August) 40 r~ O 20 1991 (July) 4O Z//H l/H/ 20 1992 (June) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 subregion Fig. 2. Spatial distribution of benthic macroalgae in the intertidal area of Niedersachsen (Germany) 1990-1992 Species composition Already during the first outburst of macroalgal growth in 1989, samples for tax- onomic determinations were collected in July from 16 locations near Norderney and Baltrum (subregions 5 and 6). The material was completed by collections in July 1991 522 K. Kolbe, E. Kaminski, H. Michaelis, B. Obert & J. Rahmel from 7 locations (subregions 1, 8, 12) and July 1992 from 8 locations (subregion 5). From each station a random collection, partly preserved (4 % formalin in seawater) and partly fresh thalli, was determined to species and, in some cases, to subspecies level. Descriptions of substrata and the effects of macroalgae on sediment and benthos are based on quahtative field observations. RESULTS Spatial, seasonal and interannual variability of coverage As an example, Figure 3 represents the results of two surveys in the tidal flat region behind four of the East Frisian Islands in July and August 1991. The location and extension of the algal fields are given without indication of cover densities. Not only the size of the algal fields changed from one month to another, but also their position was subject to considerable shirtings. Figure 2 illustrates how the maximum coverage of algal mats was distributed over the 16 subregions of the survey area in the years 1990-92. Only the two cover categories <50 % and >50 % are considered. The areas of dense algal cover shifted from the western and middle subregions in 1990, to the middle and eastern subregions in 1991 and 1992. In 1990 (August) the maximum coverage was recorded on the tidal flats of the western-most East Frisian islands Borkum (subregion 1) and Juist (subregion 2), with 60 % and 40 % coverage, respectively; however, high densities of algae also occurred in the intertidal of the other islands (subregion 5-9). This was the most intense algal development observed within the three years. In 1991 (July) and 1992 (June), the area between Norderney and Wangerooge (subregion 5-9) supported the major part of the algal crop. Furthermore, the tidal flats between the Jade and Elbe estuary (subregion 11-16) were more severely affected than in 1990. With the exception of the long-lived mussel beds and some Lanice-populations which were covered annually, the vegetation pattern in each subregion changed every year. Constantly low algal cover was registered in subregion 4: a sheltered, muddy bay without suitable substrata for macroalgae. The seasonal dynamic of the macroalgal development as derived from the aerial surveys differed from year to year, with regard to both the onset of growth, and the duration of the vegetation period. In 1990 (5 monthly surveys), growth started early in May, and the mats largely survived until late September. In 1991 (3 monthly surveys) the growing period began in June and most of the coverage had already disappeared by late August. In 1992 (5 monthly surveys), algal development recommenced in May, but - different from 1990 - the stands became rather scarce by September. The peak of algal cover was recorded in late August for 1990, in July for 1991 and for 1992 already in June. The results of aerial surveys during the times of maximum extension (Table 1), give an impression of the interannual variability of algal cover. The total area (cover density 1-100 %) was approximately the same size every year: about 180 km 2 or 15 % of the surveyed area, respectively. However, with regard to the vegetation density, distinct interannual differences can be defined. The major coverage was recorded in 1990, when almost 70 % of the overgrown area supported an algal cover of the categories 4 or 5 (>50 %). In 1991 and 1992, only some 20 % of the vegetation was that dense, In 1991, Macroalgae in the Wadden Sea 523 July7th-10th 1991 LAN( fj~~NORDERNEYBALTRUM ,- "~t~ . :.; Scattered thalliof algae /~)-r ~ Algae stands August22nd-29th 1991, (,' ~~ ~y 0 ; l'0km Fig. 3. Two examples of aerial survey results in the East Frisian part of the Wadden Sea. Areas covered by algae are given without indication of different cover densities only approximately 50 % of the area involved was more than 20 % covered. In 1992, the 20 % cover category was surpassed by 70 % of the algal mats. Species composition, substrata preference, and effects In addition to the results of the aerial surveys, only preliminary and tentative ground truth data are available, with hardly any information on spatial and seasonal variation. Sampling for taxonomy and observations of substrata and effects were restricted to only a few spots in the intertidal area of the Niedersachsen coast. 524 K. Kolbe, E. Kaminski, H. Michaelis, B. Obert & J. Rahmel Table 1. Maximum macroalgal cover recorded by aerial surveys in the summers of 1990-1992 on the intertidal flats of Niedersachsen (Germany) Cover category Area covered (kin 2) 1990 1991 1992 (August) (July) (June) 0 % 1040 1040 1058 1- 5% 11 13 24 6- 20 % 26 84 36 21- 50 % 22 53 70 51- 80% I25 34 37 81-100% 3 3 2 total cover 187 187 169 % share of surveyed area 15 % 15 % 14 % Concerning species composition, predominantly representatives of the family Ulvaceae were identified in the algal mats.